1. Field of the Invention
The present invention relates to a semiconductor laser device in which an active layer having a quantum well structure is sandwiched between a lower optical confinement layer and an upper optical confinement layer, and a method of manufacturing the semiconductor laser device by using the metalorganic chemical vapor deposition method (MOCVD method). More particularly, the present invention relates to a semiconductor laser device in which the composition of the semiconductor material constituting the upper and lower optical confinement layers is continuously changed in a thickness direction of the layers, and a method of manufacturing the semiconductor laser device by using the MOCVD method.
2. Prior Art
In a laser device in which a multiple quantum well (MQW) structure is adopted in the active layer, a threshold current is at a low level, and high optical power operation is possible. And in general, the upper optical confinement layer and the lower optical confinement layer each having the SCH structure (separate-confinement-heterostructure) are provided on both (upper and lower) surfaces of the active layer by means of the heterojunction, thereby improving the carrier injection efficiency and the confinement effect of laser light oscillated in the active layer. Thus, the external differential quantum efficiency of the laser device is enhanced and the high optical power operation can be achieved.
As SCH structures for use in such a case, the following structures are designed. That is, the SCH structure obtained by stacking the layers made of the same kind of semiconductor materials having different composition ratios by means of the heterojunction while changing the composition ratios by stages, and the SCH structure obtained by stacking the same kind of semiconductor materials while continuously changing the composition ratios thereof have been designed.
Of these SCH structures, if the SCH structure in which the composition of the material thereof is continuously changed in the thickness direction is employed, the carrier injection efficiency to the active layer is enhanced, and larger optical confinement effect can be obtained. In addition, it is known that since the optical confinement layer does not contain the heterojunction interface causing the crystal degradation, the advantages that the reliability at the time of the high optical power operation can be improved can be obtained.
A laser device A serving as an example of the laser device in which the lower optical confinement layer and the upper optical confinement layer with the latter SCH structure are formed is shown in FIG. 1. Also, a diagram representing a conventional energy band in a layered structure C in the laser device A is shown in FIG. 2.
In this laser device A, a lower cladding layer 2A with a thickness of 500 nm and made of n-InP is stacked on a substrate 1 made of, for example, n-InP. On the lower cladding layer 2A, a lower optical confinement layer 3A made of InGaAsP, an active layer 4 with the MQW structure made of InGaAsP/InGaAs, an upper optical confinement layer 3B made of InGaAsP, and an upper cladding layer 2B with a thickness of 500 nm and made of p-InP (all of them will be described later) are sequentially stacked to form the layered structure C.
Note that a current blocking layer 6 consisting of a p type layer 6B and an n type layer 6A sequentially stacked is formed on both sides of the layered structure C.
Then, an upper cladding layer 2C is formed so as to bury the layered structure C and the current blocking layer 6, and a cap layer 5 made of p-InGaAsP with a thickness of 50 nm is further stacked thereon. An upper electrode 7B is formed on the cap layer 5 and a lower electrode 7A is formed on the rear surface of the substrate 1.
In the layered structure C described above, the active layer 4 is designed in the following manner.
That is, in the active layer 4, a well layer 4A is constituted of an InGaAsP layer with a thickness of 4 nm, a barrier layer 4B with a thickness of 10 nm is formed of InGaAsP with a composition having a bandgap wavelength of 1.2 xcexcm, and a total of five quantum wells are provided (refer to FIG. 2).
On the other hand, the lower optical confinement layer 3A and the upper optical confinement layer 3B are designed in such a manner as follows.
That is, the thickness of the lower optical confinement layer 3A and the upper optical confinement layer 3B is set at 40 nm. With respect to the lower optical confinement layer 3A, a heterojunction part (1) with the lower cladding layer 2A is made of InGaAsP with a composition having a bandgap wavelength of 0.92 xcexcm, and a heterojunction part (2) with the first well layer 4A of the active layer 4 is made of InGaAsP with a composition having a bandgap wavelength of 1.2 xcexcm.
Furthermore, in the region between the part (1) and the part (2), the bandgap wavelength is sequentially increased from 0.92 xcexcm to 1.2 xcexcm. More specifically, this part of the layer is formed by the sequential stack of the InGaAsP with such a composition that the bandgap energy is sequentially decreased and the refraction index is sequentially increased.
Thus, as shown in FIG. 2, the lower optical confinement layer 3A is formed of InGaAsP with such a graded composition that the bandgap wavelength thereof is linearly increased from the lower cladding layer 2A to the first well layer 4A of the active layer 4.
Also, the configuration of the upper optical confinement layer 3B is designed to be reversal to that of the lower optical confinement layer 3A with respect to the active layer 4 serving as the center thereof.
That is, the heterojunction part with the last well layer 4A of the active layer 4 is formed of InGaAsP with a composition having a bandgap wavelength of 1.2 xcexcm, and the heterojunction part with the upper cladding layer 2B is formed of InGaAsP with a composition having a bandgap wavelength of 0.92 xcexcm. Thus, the layer between the parts is formed of InGaAsP with such a graded composition that the bandgap wavelength is sequentially and linearly decreased.
In the manufacture of the above-mentioned laser device, the MOCVD method is usually employed. For example, TMIn (trimethylindium) is used as In source, TMGa (trimethylgallium) is used as Ga source, AsH3 (arsine) is used as As source, and PH3 (phosphine) is used as P source. Then, these gas sources are diluted with H2 to a predetermined concentration, and these gas sources are subjected to accurate flow rate control and time control by means of the mass flow controller in accordance with the kind of the semiconductor layers to be formed, then they are supplied to a reactor, and thus, sequentially forming predetermined semiconductor layers.
For example, the above-described lower optical confinement layer 3A in the layered structure C can be formed in such a manner as follows.
After the process of forming the lower cladding layer made of n-InP by the use of TMIn (In source), PH3 (P source), and n type impurity gas source, the supply of the n impurity gas source is stopped. Next, while maintaining the supply of the In source and the P source, the mass flow controller of TMGa (Ga source) and that of AsH3 (As source) are opened to start the supply of the Ga source and the As source to the reactor.
Then, the openings of the valves of the mass flow controllers of the In source and the P source are controlled to gradually reduce the supply flow rate thereof, and the supply flow rates of the Ga source and the As source are gradually increased from 0 by controlling the valves of the mass flow controllers thereof. Note that the supply flow rates of these gas sources are controlled to a certain value so that the composition of the InGaAsP layer formed at each time can be equal to the composition having a designed bandgap wavelength shown in FIG. 2.
Through the operations as described above, the lower optical confinement layer 3A made of InGaAsP, in which the composition ratio of In, Ga, As, and P is continuously changed is formed on the lower cladding layer 2A.
Note that the upper cladding layer 3B can be formed by the operations reverse to those of the lower cladding layer 3A after forming the active layer 4.
FIG. 3 shows the result of the secondary ion mass-spectroscopy (SIMS) for the layered structure obtained in the fabrication of the laser device through the above-described operations using the MOCVD method, which is designed so as to achieve the energy band diagram shown in FIG. 2.
In FIG. 3, the vertical axis represents the number of counts of the secondary ions of each element, and the horizontal axis represents the positions of the respective layers in the layered structure C.
As is apparent from the SIMS curve shown in FIG. 3, the number of counts of the secondary ions with respect to the Ga source and the As source are unstably fluctuated in the regions of the small supply flow rates thereof.
More specifically, it has been found out from FIG. 3 that, in the case of the crystal growth of mixed crystals having a certain composition by the MOCVD method, the composition ratio of the component elements in the formed crystal layer is unstable during the time when the supply flow rate of a component element (As in the case of FIG. 3) is small.
Particularly, in the case of the SIMS curve relative to As, the number of counts become remarkably unstable in an early stage S1 in the formation of the lower optical confinement layer 3A. More specifically, the number of counts does not smoothly increase but significantly fluctuate in the lower optical confinement layer 3A. And then, the SIMS curve becomes steep at the time when the crystal growth reaches a certain thickness as indicated by S3 in FIG. 3.
In addition, the number of counts of As secondary ions sharply decreases at the point S2 close to the junction between the upper optical confinement layer 3B and the upper cladding layer 2B.
It can be understood from the foregoing description that the stable formation of the crystal layer in line with the design cannot be achieved. Accordingly, the improvement in the carrier injection efficiency and the crystallinity intended in the design stage cannot be expected. On the contrary, the deterioration in such characteristics may be caused.
In addition, when such situations occur, the refraction index of the upper and lower optical confinement layers deviates from the design standard, and the condition in the optical confinement in the active layer is disadvantageously changed.
An object of the present invention is to provide a semiconductor laser device provided with a layered structure in which composition ratio in the formed crystal layer is stable even in a region where a supply flow rate of a component element is small, and improvement in the carrier injection efficiency and crystallinity can be achieved.
Another object of the present invention is to provide a method of manufacturing the semiconductor laser device.
In order to achieve the objects, the present invention provides a semiconductor laser device, provided with a layered structure in which a lower cladding layer, a lower optical confinement layer, an active layer having a quantum well structure, an upper optical confinement layer, and an upper cladding layer are stacked in this order on a substrate,
wherein both the lower optical confinement layer and the upper optical confinement layer are made of mixed crystals of compound semiconductors with a composition continuously changed in a thickness direction, and
a crystal layer (hereinafter, referred to as an interposition layer and referred to as a minute flow rate controllable layer in claims) obtained as a result of the control of the supply flow rate of the minute gas source is interposed in at least one of the interfaces between the lower cladding layer and the lower optical confinement layer, between the lower optical confinement layer and the active layer, between the active layer and the upper optical confinement layer, and between the upper optical confinement layer and the upper cladding layer.
In a preferable aspect, the present invention provides a semiconductor laser device in which both the lower optical confinement layer and the upper optical confinement layer are made of the four-element mixed crystals with a composition continuously changed in a thickness direction, and a layer made of the four-element mixed crystals with a composition having a bandgap wavelength of 0.93 to 1.05 xcexcm is interposed at least between the lower optical confinement layer and the lower cladding layer.
In addition, the present invention provides a method of manufacturing a semiconductor laser device, which is provided with a layered structure in which a lower cladding layer, a lower optical confinement layer made of mixed crystals of compound semiconductors with a composition continuously changed in a thickness direction, an active layer having a quantum well structure, an upper optical confinement layer made of mixed crystals of compound semiconductors with a composition continuously changed in reverse to that of the lower optical confinement layer with respect to the active layer, and an upper cladding layer are stacked in this order on a substrate,
wherein each of the layers in the layered structure is formed by the metalorganic chemical vapor deposition apparatus, in which the source gases are supplied to the reactor while controlling the supply flow rates of the gas sources of the component elements by the mass flow controller, and
in the case where the supply flow rate of the gas source is smaller than the controllable limit of the flow rate of the mass flow controller, the method includes the step of: supplying the gas source previously flowed in the exhaust path with the flow rate controllable by the mass flow controller to the reactor, alternatively, stopping the supply of the gas source and then supplying gas source of the component element of the layer formed in the next process to the reactor.
More specifically, the present invention provides a method of manufacturing a semiconductor laser device (hereinafter, referred to as the first manufacturing method), in which, the gas source of the element which is not used in the formation of the lower cladding layer is previously flowed in the exhaust path until the time when the formation of the lower optical confinement layer is started, and at the same time of the start of the formation of the lower optical confinement layer, the gas source is supplied to the reactor.
Alternatively, the present invention provides a method of manufacturing a semiconductor laser device (hereinafter, referred to as the second manufacturing method), in which, at the time reaching the completion of the formation of the lower optical confinement layer and when the flow rate of the supplied gas source is smaller than the controllable limit of the flow rate of the mass flow controller, the supply of the gas source is stopped, and then, the gas sources of the component elements of the active layer are supplied to the reactor.